In recent years, the JATMA (Japan Automobile Tyre Manufacturers Association) labeling (display) system has been introduced for pneumatic tires for passenger cars, and such tires require superior levels of both low rolling resistance and wet grip performance. In particular, the required level of wet grip performance is extremely high, and a pneumatic tire able to achieve grade a in terms of wet grip performance based on this labeling system has yet to be developed. At the same time, improvement in wear resistance has been demanded in order to extend tire life.
It is known that, conventionally, silica is blended in rubber compositions constituting tread portions of pneumatic tires in order to enhance the balance between low rolling resistance and wet grip performance. However, silica readily aggregates due to the presence of surface silanol groups, and because silica exhibits poor affinity for diene rubber, silica dispersibility can be poor and, in such cases, the effect of modifying the dynamic visco-elasticity properties, such as the loss tangent (tan δ), of a rubber composition cannot be satisfactorily achieved. In addition, there is another problem that reinforcing effects of silica is lower than that of carbon black and wear resistance can be insufficient, and when the dispersibility is poor, the wear resistance can be further lowered.
To resolve this problem, Japanese Unexamined Patent Application Publication No. 2009-091498A proposes enhancing dispersibility of silica by a rubber composition compounding silica in solution polymerization-produced terminally modified styrene-butadiene rubber in which terminals are modified with a polyorganosiloxane or the like. Although this rubber composition exhibits effects of enhancing rolling resistance, wet grip performance, and wear resistance, levels of improvement that is demanded by consumers for low rolling resistance, wet grip performance, and wear resistance are even higher, and further improvement in balance between these performances has been demanded.